Activation of Hexose Transport by Concanavalin A in Isolated Brown Fat Cells

Direct activation of the fat cell d-glucose transport system by the lectin concanavalin A was studied using a rapid filtration procedure which monitors 3-O-[3H]methylglucose uptake in isolated brown fat cells. Transport of this non-metabolized d-glucose analogue, which occurs by facilitated diffusion, was linear for 5 or more min, temperature-dependent, and inhibited by unlabeled d-glucose, phlorizin, and cytochalasin B. Concanavalin A mimicked the stimulatory effect of insulin on initial rates of 3-O-methylglucose transport, whereas the lectin did not modify the equilibrium distribution of sugar. Marked agglutination of brown fat cells accompanied the lectin effects on transport. Experiments with l-[14C]glucose, which enters the cells by simple diffusion, indicated that the effect of concanavalin A was specific for the d-glucose transport system. Incubation of brown fat cells with 0.5 mg per ml of trypsin for 15 min at 37° almost completely inhibited the response of fat cell 3-O-methylgucose transport to insulin but had little or no effect on concanavalin A action or control transport rates. Under these conditions trypsinization had no effect on 125I-concanavalin A binding to fat cell surface receptors nor on lectin-mediated agglutination of these cells. The data are thus consistent with our previous suggestion that the physiological activity of this lectin in fat cells is mediated by receptor moieties distinct from those involved in insulin action. Digestion of cells with 200 µg per ml of neuraminidase abolished the stimulatory effects of both the lectin and insulin on 3-O-methylglucose transport. In addition, this treatment completely prevented agglutination of brown fat cells by concanavalin A without a detectable diminution of the 125I-concanavalin A bound to these cells. Bovine submaxillary mucin but not the glycoprotein conalbumin which is devoid of sialic acid blocked the actions of neuraminidase on insulin-stimulated hexose transport as well as agglutination of cells by the lectin. The data suggest that sialic acid moieties of fat cell membrane components play an important role in membrane events involved in lectin-mediated cell agglutination and hexose transport activation which occur subsequent to lectinreceptor binding. It is thus possible that the modulation of the glucose transport system by the lectin is dependent upon the same cell surface events which mediate its agglutination of cells.